Hydrocephalus is usually due to blockage of CSF outflow in the ventricles or in the subarachnoid space over the brain. Hydrocephalus treatment is surgical: it involves the placement of a ventricular catheter (a tube made of silastic for example) into the cerebral ventricles to bypass the flow obstruction/malfunctioning arachnoidal granulations and the draining of the excess fluid into other body cavities, from where said fluid can be resorbed.
Most of the CSF shunts have been based on the principle of maintaining a constant intracranial pressure (ICP) regardless of the flow-rate of CSF. The CSF shunts have been constructed to cut off CSF-flow when the differential pressure between the inlet and the outlet of the CSF shunt was reduced to a predestined level, called the opening pressure of the shunt.
An example of an ICP shunt is shown in U.S. Pat. No. 3,288,142 to Hakim, which is a surgical drain valve device used to control the drainage of fluid between different portions of the body of a patient, particularly for draining cerebrospinal fluid from the cerebral ventricles into the blood stream (co called ventriculo-atriostomy).
Clinical experience has proven that this principle of shunting is not an ideal solution. Sudden rises of the ICP, e.g. due to change of position, physical exercise, or pathological pressure waves result in excessive CSF drainage. Several reports in the literature (Aschoff et al., 1995) point at problems due to this overdrainage, and especially the pronounced narrowing of the ventricles has been pointed out as being the main factor leading to malfunctioning of the implanted shunting device. The reason is that the ventricular walls may collapse around the ventricular CSF shunt device, and particles (cells, debris) may intrude into the shunt device.
U.S. Pat. No. 5,192,265 to Drake et al. describes an example of a shunt seeking to overcome the above-mentioned difficulties by proposing a rather complex anti-siphoning device allowing to select transcutaneously the resistance to flow by controlling the pressure in a chamber gas-filled and being in pressure communication with one flexible wall of the main chamber where the flow is regulated.
The use of programmable valves was associated with a reduction in the risk of proximal obstruction and overall shunt revision, one possible explanation for a difference in the two populations studied is that programmable valves may allow the physician to avoid such ventricular collapse by increasing the valve pressure setting after noting clinical signs and symptoms and/or radiological evidence of overdrainage. In this way, proximal obstruction is prevented, and shunt revision surgery is avoided. One such adjustable valve is described in U.S. Pat. No. 4,551,128 to Hakim et al. However, due to the elastomeric properties of the diaphragm material, maintenance of the implanted valve may be required. Further, flow rate adjustment of this adjustable valve after implantation may require a surgical procedure.
Another adjustable valve mechanism, described in U.S. Pat. No. 4,781,673 to Watanabe, includes two parallel fluid flow passages, with each passage including a flow rate regulator and an on-off valve. Fluid flow through the passages is manually controlled by palpably actuating the on-off valves through the scalp. Although the Watanabe device permits flow rate control palpably through the scalp and thus, without surgical intervention, patient and/or physician attention to the valve settings is required.
One system, described in U.S. Pat. No. 6,126,628 to Nissels, describes a dual pathway anti-siphon and flow-control device in which both pathways function in concert. During normal flow, both the primary and secondary pathways are open. When excessive flow is detected, the primary pathway closes and flow is diverted to the high resistance secondary pathway. The secondary pathway decreases the flow rate by 90% while maintaining a drainage rate within physiological ranges, which prevents the damaging complications due to overdrainage. However, this device is intended for use with a shunt system including a valve for controlling flow rate and should be placed distal to the valve inducing cumbersome procedure due to the additional material to be implanted. The system can be used as a stand-alone only for low-pressure flow-control valve.
A valve made of a spring and an arrow-shaped piston moving into a hollow cylinder is described in Patent application EP 0414 649 A2. By increasing the inlet pressure, the piston is moving against the antagonist spring in the cylinder. The piston is made of a conical head 8, a first cylindrical section 11 having a first diameter, a second cylindrical section 10 having a second diameter smaller than the first one and showing a through hole 13 and an internal channel 12, a third cylindrical section 9 having a diameter equal to the first diameter, and finally a last cylindrical section 14 having a diameter small than the first diameter. The first cylindrical section 11 is used to guide the piston 6 in the cylinder 2. The fluid is able to flow in the restriction between the section 11 and the cylinder 2 since the cylinder 2 has a seat 5 with an enlarging triangular shape as shown FIGS. 4 and 5. The fluidic restriction can be seen as a bowed channel having a constant length (equal to the width of the section 11) but a section that increases with the pressure since this section is defined by the triangular shape (5). The depth of the bowed channel is defined by the thickness of the hollow cylinder (2). It is important to note that the main fluidic resistance of the device is said bowed channel has a constant length. The length is defined here according to the Poiseuille's law, i.e. by reference to the direction of the flow. The section of the channel is therefore defined by the normal to the flow direction.
The section 10 is used to collect the fluid via the hole 13 and the internal channel 12 up to the outlet of the device. The channel 12 has a length and a section that do not change with the applied pressure.
The section 9 has only a guiding function and no fluidic action thanks to the large internal channel 12.
By increasing the pressure the fluidic resistance of said channel decreases. By plotting the flow rate against the applied pressure, the slope of the curve is therefore increasing with the pressure, leading to a behaviour similar to other hydrocephalic valves (e.g. Spitz-Holter). This device can be seen as a pressure regulator having a shutoff valve function associated in series with a flow regulator.
Kuffer et al., U.S. Pat. No. 3,674,050 (1972), shows a valve having also a spring, but the fluidic resistance of the channels 10 in FIG. 2 does not vary with the applied pressure.
Griswold, U.S. Pat. No. 3,752,183 (1973), shows a valve having a piston having a shoulder to support the spring action. The piston is not guided along its whole length.
Hooven, U.S. Pat. No. 4,675,003 (1987), shows another hydrocephalus valve having springs but no cylindrical piston.
Christopher, U.S. Pat. No. 4,682,625, provided a shutoff valve having a spring and a cylinder, wherein said cylinder showed several diameters and several guiding areas. The principle is very similar to Patent application EP 0414 649 A2 mentioned above, except for the progressive opening of the fluidic pathway. The main fluidic resistances, i.e. between 120 and 64 or the openings 142 and 144 do not vary with the applied pressure. There is only a change of section from 40 to 64 leading to the opening of the valve after a given pressure threshold that depends on the spring stiffness.
Scholin, U.S. Pat. No. 2,977,980, shows another unidirectional valve very similar to Christopher (U.S. Pat. No. 4,682,625), having a piston, a cylinder having several diameters and a spring acting on a shoulder of the piston. Once the valve is open, according to Christopher, the fluidic resistance of the device do not depends on the applied pressure.
Roland, French Patent FR 2 685 206 A (Cordis SA), shows a device for hydrocephalus treatment made of a membrane 34 having a calibrated hole 39 that connects the upstream chamber 35 and the downstream chamber 36. The hole 39 is bordered by seals 40 and 41. At low pressure the seal 40 is in contact with the seat 42 inducing a blocking of the flow. By increasing the pressure, the membrane is pushed downward and the fludic pathway is opened between 40 and 42 up to the chamber 36. The distance between the seal 41 and the seat 44 decreases, inducing an annular flow restriction. This annular flow restriction has a fluidic resistance that changes with the applied pressure. The flow restriction can be seen as an annular channel of a constant length (with respect to the flow direction) but having a height decreasing with the pressure. According to the Poiseuille's law, the fluidic resistance of an annular ring varies as the power −3 with the distance between the seal 41 and the seat 44 as for any flat channel while the deflection of the elastic membrane varies linearly with the applied pressure. By design, such device cannot exhibit constant flow rate regulation even by considering two valves in series. Moreover the design is very sensitive to the membrane thickness and tolerances. The use of a spring that can be tested and calibrated before assembly should provide better accuracy than the flexible membrane proposed here. The device does not exhibit guided piston. According to the state of the art, a slit or ball valve having a preloaded spring has been proposed here to regulate the pressure. The principle of a pressure regulator in series with a flow regulator has been already proposed by EP Patent application EP 0 414 649 A2 as discussed previously. The principle of flow regulation by closing gradually a channel has been also proposed by Park which reported a constant flow-rate microvalve for hydrocephalus treatment [S. Park, W. H. Ko, and J. M. Prahl, “A constant flow-rate microvalve actuator based on silicon and micromachining technology,” in Tech. Dig. 1988 Solid-State Sens. Actuator Workshop (Hilton Head '88), Hilton Head Island, S.C., Jun. 6-9 (1988) 136-139]. The valve is made of a diaphragm covering a flat substrate; the channel cross-section diminishes under increasing pressure, thus leading to quasi-steady flow-rate. Here again, both theoretical and experimental data reported show that a perfectly steady rate cannot be achieved since the flow resistance should increase with the applied pressure in a linear manner while the variation of the channel cross-section is strongly non-linear. It is clear that the control of the channel shape is fundamental to get an accurate and reproducible flow regulation.
Leonhardt et al., Patent application EP 0 982 048 A1, proposes several methods to monitor intracranial pressure and cerebrospinal fluid flow rate for implantable devices having an active actuator.